Casting of an aluminium alloy

ABSTRACT

A casting with good heat resistance comprises an alloy with 2 to 4 w. % magnesium 0.9 to 1.5 w. % silicon 0.1 to 0.4 w. % manganese 0.1 to 0.4 w. % chromium max. 0.2 w. % iron max. 0.1 w. % copper max. 0.2 w. % zinc max. 0.2 w. % titanium max. 0.3 w. % zirconium max. 0.008 w. % beryllium max. 0.5 w. % vanadium with aluminium as the remainder, with further elements and production-induced contaminants individually max. 0.02 w. %, total max. 0.2 w. %.

The invention concerns a casting of an aluminium alloy with good heat resistance.

For thermally stressed components today normally AlSi alloys are used, where the heat resistance is achieved by the addition of Cu to the alloy. Copper, however, also increases the heat crack tendency and has a negative effect on the castability. Applications in which particular heat resistance is required normally occur in the field of cylinder heads in automobile construction, see e.g. F. J. Feikus, “Optimisation of Aluminium Silicon Casting Alloys for Cylinder Heads”, Giesserei-Praxis 1999, Vol. 2, pages 50-57.

WO-A-0043560 discloses an aluminium alloy with 2.5-7.0 w. % Mg, 1.0-3.0 w. % Si, 0.3-0.49 w. % Mn, 0.1-0.3 w. % Cr, max. 0.15 w. % Ti, max. 0.15 w. % Fe, max. 0.00005 w. % Ca, max. 0.00005 w. % Na, max. 0.0002 w. % P, other contaminants individually max. 0.02 w. % and aluminium as the remainder, for the production of safety components in diecasting, squeeze casting, thixoforming and thixoforging processes.

The invention is based on the object of preparing an aluminium alloy with good heat resistance suitable for the production of thermally stressed components. The alloy is particularly suitable for gravity diecasting, low pressure chilled casting and sand casting.

Components cast from the alloy should have a high strength in connection with high ductility. The desired mechanical properties of the component are defined as follows: Yield strength Rp0.2 > 170 MPa Tensile strength Rm > 230 MPa Elongation at fracture A5 > 6%

Because of the applications, the corrosion tendency of the alloys should be kept as low as possible and the alloy must have a correspondingly good fatigue strength. The castability of the alloy should be better than that of the AlSiCu casting alloys which are currently used, and the alloy should have no tendency to heat cracks.

The term “casting” includes, as well as the pure components produced solely by casting, those cast as a premould and subsequently formed to the final dimensions by hot or cold shaping.

Examples of pure castings are those which are produced exclusively by sand casting, gravity diecasting, low pressure chilled casting, diecasting, thixocasting or squeeze casting.

Forming operations performed on a cast premould by shaping are for example forging and thixoforging.

The object according to the invention is achieved by an aluminium alloy with

-   2 to 4 w. % magnesium -   0.9 to 1.5 w. % silicon -   0.1 to 0.4 w. % manganese -   0.1 to 0.4 w. % chromium -   max. 0.2 w. % iron -   max. 0.1 w. % copper -   max. 0.2 w. % zinc -   max. 0.2 w. % titanium -   max. 0.3 w. % zirconium -   max. 0.008 w. % beryllium -   max. 0.5 w. % vanadium -   with aluminium as the remainder, with further elements and     production-induced contaminants individually max. 0.02 w. %, total     max. 0.2 w. %.

The following content ranges are preferred for the individual alloy elements: Mg 2.5 to 3.5 w. %, in particular 2.7 to 3.3 w. % Si 0.9 to 1.3 w. % Mn 0.15 to 0.3 w. % Cr 0.15 to 0.3 w. % Ti 0.05 to 0.15 w. % Fe max. 0.15 w. % Cu max. 0.05 w. % Be 0.002 to 0.005 w. % V 0.01 to 0.1 w. % Zr 0.1 to 0.2 w. %

The effect of the alloy elements can be characterised approximately as follows:

Silicon in conjunction with magnesium leads to a corresponding hardening where in particular thermal hardening is of interest. Preferred is heat treatment to a state T6 e.g. solution annealing at 550° C. for 12 hours with subsequent artificial ageing at 160-170° C. for 8 to 10 hours.

The combination of manganese and chromium leads to good heat resistance at a sustained temperature of up to 180° C.

Titanium and zirconium are used for grain refining. Good grain refining makes a substantial contribution to an improvement in casting properties.

Beryllium in conjunction with vanadium reduces the dross formation.

A preferred area of application of the castings according to the invention is thermally stressed components, in particular pressure vessels, compressor housings and engine components such as cylinder heads in automobile construction. The components are preferably produced in the sand casting or chilled casting process.

Further advantages, features and details of the invention arise from the description below of preferred embodiment examples and the drawing which shows:

FIGS. 1-3 tensile strength, yield strength and elongation at fracture as a function of temperature after 500 hours sustained temperature load for an alloy according to the invention and a comparison alloy according to the prior art.

An alloy according to the invention reference AlMg3SilMnCr and a comparison alloy reference AlSi7MgCu1 by F. J. Feikus, “Optimisation of Aluminium Silicon Casting Alloys for Cylinder Heads”, Giesserei-Praxis 1999, Vol. 2, pages 50-57, with the compositions given in table 1, were compared with regard to long-term behaviour under sustained temperature load. TABLE 1 Chemical Composition of Alloys (in w. %) Alloy Si Fe Cu Mn Mg Cr Zn Ti Be V Zr AlSi7MgCu1 6.97 0.11 0.94 0.005 0.38 0.008 0.03 AlMg3Si1MnCr 1.10 0.07 0.001 0.20 3.2 0.21 0.002 0.12 0.003 0.03 0.0005

The alloy according to the invention was cast in a trial rod mould according to Diez for round rods 16 mm diameter. The mechanical properties of yield strength (Rp0.2), tensile strength (Rm) and elongation at fracture (A5) were determined on the trial rods in state T6 (165° C./6 hours) after a sustained temperature load of 500 hours at various temperatures. The corresponding values for the comparison alloy were taken from the above article by F. J. Feikus. The results are shown in FIG. 1 in diagram form.

The alloy AlMg3Si1MnCr according to the invention admittedly does not reach the peak values of the comparison alloy AlSi7MgCu1 with regard to yield strength and tensile strength, but in its temperature behaviour is “less changeable”. This changeability has a disruptive effect in operation insofar as slight changes in temperature can cause great changes in mechanical properties. The yield strength of the alloy according to the invention remains at around the same level up to around 180° C., gradually falls away up to 200° C., and only above around 200° C. begins to decrease continuously. The continuous decrease takes place with a lesser gradient than the alloy AlSi7MgCu1.

With regard to the elongation at fracture, the alloy according to the invention is characterised by an almost constant value up to 180° C. High elongation values give a favourable fracture/failure behaviour. A visible deformation precedes the break of the component. Above 180° C. the elongation rises continuously. In the comparison alloy AlSi7MgCu1, the clear hardening effect can be seen. Low elongation values cause an unfavourable failure behaviour i.e. the component only deforms slightly or not at all. Under load peaks the component breaks without warning. 

1: A casting process for producing a cast product comprising: (a) providing an aluminum alloy comprising: 2 to 4 w. % magnesium 0.9 to 1.5 w. % silicon 0.1 to 0.4 w. % manganese 0.1 to 0.4 w. % chromium max. 0.2 w. % iron max. 0.1 w. % copper max. 0.2 w. % zinc max. 0.2 w. % titanium max. 0.3 w. % zirconium max. 0.008 w. % beryllium max. 0.5 w. % vanadium with aluminium as the remainder, with further elements and production-induced contaminants individually max. 0.02 w. %, total max. 0.2 w. %; and (b) casting the aluminum alloy to produce a cast product. 2: A process according to claim 1, wherein the alloy contains 2.5 to 3.5 w. % Mg. 3: A process according to claim 1, wherein the alloy contains 0.9 to 1.3 w. % Si. 4: A process according to claim 1, wherein the alloy contains 0.15 to 0.3 w. % Mn. 5: A process according to claim 1, wherein the alloy contains 0.15 to 0.3 w. % Cr. 6: A process according to claim 1, wherein the alloy contains 0.05 to 0.15 w. % Ti. 7: A process according to claim 1, wherein the alloy contains max. 0.15 w. % Fe. 8: A process according to claim 1, wherein the alloy contains max. 0.05 w. % Cu. 9: A process according to claim 1, wherein the alloy contains 0.002 to 0.005 w. % Be. 10: A process according to claim 1, wherein the alloy contains 0.01 to 0.1 w. % V. 11: A process according to claim 1, wherein the alloy contains 0.1 to 0.2 w. % Zr. 12: A process according to claim 1, wherein the alloy is sandcast or chilled casting process. 13: A process according to claim 11 wherein the alloy is chillcast or chilled casting process. 14: An aluminium alloy with good heat resistance comprising: 2 to 4 w. % magnesium 0.9 to 1.5 w. % silicon 0.1 to 0.4 w. % manganese 0.1 to 0.4 w. % chromium max. 0.2 w. % iron max. 0.1 w. % copper max. 0.2 w. % zinc max. 0.2 w. % titanium max. 0.3 w. % zirconium max. 0.008 w. % beryllium max. 0.5 w. % vanadium with aluminium as the remainder, with further elements and production-induced contaminants individually max. 0.02 w. %, total max. 0.2 w. %. 15: The alloy of claim 14, wherein the alloy contains 2.7 to 3.3 w. % Mg. 16: The alloy of claim 14, wherein the alloy contains 0.05 to 0.15 w. % Ti. 17: The alloy of claim 14, wherein the alloy contains max. 0.15 w. % Fe. 18: The alloy of claim 14, wherein the alloy contains max. 0.05 w. % Cu. 19: The alloy of claim 14, wherein the alloy contains 0.002 to 0.005 w. % Be. 20: The alloy of claim 14, wherein the alloy contains 0.01 to 0.1 w. % V. 